Diagenetic evolution of Aptian evaporites in the Namibe Basin (south-west Angola) LAURENT GINDRE-CHANU*, JOHN K. WARREN † , CAI PUIGDEFABREGAS ‡ , IAN R. SHARP*, DAVID C. P. PEACOCK*, ROGER SWART § , RAGNAR POULSEN ¶ , HERCINDA FERREIRA** and LOURENCO HENRIQUE** *Statoil A.S.A., Sandsliveien 90, 5254 Sandsli, Postboks 7200, 5020 Bergen, Norway (E-mail: lgin@statoil.com) †Petroleum Geoscience Program, Department of Geology, Faculty of Science, Chulalongkorn University, 254 Phayathai Road, Pathumwan, Bangkok 10330, Thailand ‡NTNU, Høgskoleringen 5, 7034 Trondheim, Norway §Statoil A.S.A., Belas Business Park, via A1, Luanda sul, Angola ¶BlackGold Geosciences CC, PO Box 24287, Windhoek, Namibia **Sonangol A.S. Direcc ß ~ ao de Explorac ß ~ ao (DEX), Rua Rainha Ginga n.29/31, C.P. 1316, Luanda Associate Editor – Adrian Immenhauser ABSTRACT The widespread and dissected nature of the Angolan gypsiferous salt residu- als offers a uniquely detailed view of the lateral and vertical relations inhe- rent to secondary evaporite textures, which typify exhumed salt masses worldwide. Such secondary textures are sometimes misinterpreted as pri- mary evaporite textures. Thin, metre-scale and patchy, dome-like gypsum accumulations are well-exposed within strongly incised present-day river valleys along the eastern margin of the Namibe and Benguela basins (south- west Angola). These sections are time equivalent to the main basinward sub- surface evaporites (Aptian Loeme Formation) which mostly consist of halite. The gypsum (here called the Bambata Formation) is interpreted to represent the final residual product of fractional dissolution and recrystallization of the halite mass that occurred during Late Cretaceous margin uplift and con- tinues today. This halite underwent multiple episodes of diagenetic alter- ation between its deposition and its final exhumation, leading to the formation of various secondary gypsum fabrics and solution-related karst and breccia textures that typify the current evaporite outcrop. Four different diagenetic gypsum fabrics are defined: thinly bedded alabastrine, nodular alabastrine, displacive selenite rosettes and fibrous satin-spar gypsum. Current arid conditions are responsible for a thin weathered crust developed at the top of the outcropping gypsum, but the fabrics in the main core of the cur- rent at-surface evaporite unit mostly formed during the telogenetic stage of uplift prior to complete subaerial exposure. Alteration occurred as various dissolving and rehydrating saline minerals encountered shallow aquifers in the active phreatic and vadose zones. Geomorphological and petrographic analyses, mostly based on the cross-cutting relations and crystallographic patterns in the outcrop, are used to propose a sequence of formation of these different fabrics. Keywords Angola, Aptian, Bambata Formation, diagenetic gypsum, disso- lution, karstification, Namibe basin, South Atlantic. 204 © 2014 The Authors. Sedimentology © 2014 International Association of Sedimentologists Sedimentology (2015) 62, 204–233 doi: 10.1111/sed.12146